This invention relates to electric circuit breakers, and more particularly to an overcurrent protection device for use with an electric circuit breaker.
FIG. 1 is a circuit diagram of a prior art electric circuit breaker.
A three-phase power source 30 is applied to a three-phase load 32 by three lines 34. Contacts 36 are interposed in the lines between the power source 30 and the load 32. Three-phase load current through the lines 34 is detected by three current transformers 38. The output current of the current transformers 38 is rectified by full-wave diode rectifier 40. The output current of the rectifier 40 flows through a zener diode 42 and a resistor 44. The terminal voltage of the resistor 44 is applied to a control circuit 46. The output of control circuit 46 is applied to the base electrode of a transistor 48. A trip coil 50 for breaking the contacts 36 is connected in series with this transistor. The output current of the rectifier 40 is also applied to a power circuit 52 which supplies power to the control circuit 46.
When an overcurrent flows through the lines 34, a greater voltage appears across resistor 44. The control circuit 46 detects the input voltage as being abnormal, and provides an output signal to the base electrode of the transistor 48. The trip coil 50 is energized, and the contacts 36 are opened.
The current transformer 38 is thus used as a detector for detecting overcurrent conditions. The rated load current is, for example, three thousand amperes and the secondary rated current is normally fifty mA. Normally, each line 34 in the vicinity of the transformer 38 is formed as a bus bar. The primary windings of the current transformer 38 are thus constituted by the bus bars 34, so that the number of turns of each primary winding is one, and the number of turns of the secondary winding is, for example, sixty thousand. The resistance of the secondary winding is accordingly very large, as is the power consumption of the current transformer. Consequently, the secondary current of the current transformer is not proportional to the primary current when the load current is more than four or five times the rated current. Preferably, the secondary current ought to be proportional to the primary current even when the load current is ten to sixteen times the rated current so that the current transformer accurately detects the magnitude of even large overcurrent conditions. Additional problems with the use of this current transformer are that its manufacturing cost is high, because of the great number of turns required, and it also takes up a large space.
On the other hand, the current normally flowing through the secondary winding of the current transformer can be made larger by reducing the number of turns of the secondary winding. Since a resistive load is connected to the secondary winding, the power consumption of the secondary winding is I.sup.2 R. Since large load current can flow through the lines 34, power consumption in the secondary circuits can become quite large. Accordingly, the iron core of the current transformer must be made large. Should a short circuit occur, however, large undesirable overvoltages can be induced at the secondary winding. Consequently, the current transformer is designed to have a large number of turns in the secondary winding so that the secondary current is kept small.